In the field of medicine and biology or the like, in order to contribute to the clarification of biological function and pathology, it is extremely important to explore a microstructure in the living body. An optical microscope is frequently used for the observation of such biological sample. However, in the observation by the optical microscope, structure below half of a wavelength of illumination light cannot be observed due to the wave nature of light. Therefore, only not less than about 200 nm can be observed (NPL 1). For example, spatial resolution can be improved by using a laser beam of a shorter wavelength for illumination light. However, since there is a technical limit in reducing the wavelength of the laser beam, the improvement in the spatial resolution naturally has a limit.
As a method exceeding the abovementioned limit of light, there is a method suggested of using multiphoton excitation of a fluorescent material. However, since it is necessary to use light with a long wavelength for the multiphoton excitation, the substantial spatial resolution cannot be improved. Further, there are other methods suggested to exceed the limit of light using a complicated optical system or a luminescence mechanism of molecules. However, an observing object that can be applied is limited due to the particularity of a device and a material to use, it has not been successful to substantially improve the spatial resolution of the optical microscope. On the other hand, from importance of exploring the microstructure inside the living body, development of an optical observation method that can realize the spatial resolution exceeding the limit of light has been desired.
PTL 1 discloses a fluorescence microscope that can improve the spatial resolution without reducing the wavelength of the laser beam. In this fluorescence microscope, a nonlinear optical effect generated by saturation of fluorescence is used. Accordingly, it is possible to realize a fluorescence microscope with high spatial resolution without reducing the wavelength of the laser beam. However, with this fluorescence microscope, since a complicated control system must be used, introduction and employment thereof requires a great cost.
Moreover, the inventors have invented a nonlinear optical material that can improve the spatial resolution in advance of this application. This invention has been applied as Japanese Patent Application No. 2008-018665. According to this, a nonlinear optical effect can be realized by mixing and using a donor molecule and an acceptor molecule of the nonlinear optical material.